1. Field of the Invention
The present invention generally relates to time and frequency generation for communications systems and, more particularly, to low-power, low-cost and high accuracy time and frequency circuits in Global Positioning System (GPS) based tracking units.
2. Background Description
A preferred application of the present invention is the locating and tracking of assets such as rail cars, shipping or cargo containers, trucks, truck trailers, and the like, using the GPS. In such application, the GPS receivers are usually battery powered since an independent source of power is generally not available. It is advantageous to increase the operating life of the batteries by reducing the energy consumed by the GPS receiver.
In the present invention, a direct sequence spread spectrum (DSSS) communication system is used. In a typical spread spectrum receiver, the receiver front end (i.e., RF and IF electronics) consumes a large amount of power while it is turned on. This results in high energy consumption if the signal acquisition and synchronization take a long time.
A typical GPS receiver performs the following for each of at least four satellite signals:
1) acquires the DSSS signal, PA1 2) synchronizes with the message data sequence (the NAV data stream) and reads the satellite time-stamp, clock-correction, ionospheric-delay and ephemeris data, PA1 3) calculates the satellite position from the ephemeris data, PA1 4) reads its own receiver clock to determine the receiver time associated with the reception of the time-stamp epoch, and PA1 5) estimates the signal travel time by subtracting the time-stamp value from the associated receiver time.
This time difference is multiplied by the speed of light to obtain an estimated range to the satellite. If the GPS receiver had a clock that was perfectly synchronized with the clocks of the satellites (or the error was known), only three such range estimates would be required to precisely locate the receiver. There is, however, a clock-bias (slowly changing error) due the fact that GPS receivers typically use inexpensive crystal clocks, whereas the satellites are equipped with atomic clocks. This clock bias is learned and its effect eliminated by measuring the range (travel time) from four GPS satellites and using these measurements in a system of four equations with four unknowns (receiver x, y, and z, and time).
In one system in which the invention is implemented, a central facility or station must track multiple assets (e.g., railcars). Each tracked object carries a GPS receiver that processes data from several of the visible GPS satellites; however, an accurate position determination is not made at the receiver. Instead, only partial processing is done at the receiver and intermediate results are transmitted from the asset to the central station. These intermediate results do not require decoding of navigational or other data from the GPS signals. This system thus allows the GPS receiver and signal processor to be powered only long enough to acquire the satellite signals (determine the satellite/code-offset/Doppler, or SCD, bins). With this system, the dominant energy consumer is the acquisition process, and the GPS receiver energy used at each tracked asset will be dramatically reduced if the signal acquisition time and energy are dramatically reduced.